Attention has been given to lithium ion secondary batteries as a power source for driving electronic equipment. Negative electrodes for lithium ion secondary batteries comprising a graphite material have an average potential during the desorption of lithium ions of about 0.2 V (vs. Li/Li+) and exhibit a relatively flat potential. This potential is lower than that of negative electrodes comprising hard carbon, and therefore equipment that requires high voltage and voltage flatness currently employs, as the power source, lithium ion secondary batteries comprising negative electrodes including a graphite material. Graphite materials, however, have a small capacity per unit weight of 372 mAh/g, and a further increase in capacity cannot be expected.
Meanwhile, materials capable of forming an intermetallic compound with lithium are considered promising as negative electrode materials which provide a high capacity. Such materials include silicon, tin and oxides thereof. During the desorption of lithium ions, however, the crystal structure of these materials changes so that the volume of the materials increases. In the case of a negative electrode including an active material comprising Si, the negative electrode active material is represented by Li4.4Si in the state where the maximum amount of lithium ions is absorbed. When Si changes into Li4.4Si, the volume increases by 4.12 times. In the case of graphite, on the other hand, even if the maximum amount of lithium ions is absorbed, its volume increases only by 1.2 times.
A large volume change of active material results in cracking of active material particles, insufficient contact between active material and current collector, etc, which shortens charge/discharge cycle life. Particularly when cracking of active material particles occurs, the surface area of the active material particles increases, and the reaction between the active material particles and a non-aqueous electrolyte is accelerated. As a result, a film made of decomposed product of the non-aqueous electrolyte is likely to be formed on the surface of the active material. The formation of such film increases the interface resistance between the active material and the non-aqueous electrolyte, which is considered as a major cause for short charge/discharge cycle life.
In order to solve the above problem, for example, attempts have been made to form an amorphous silicon thin-film on a current collector having a rough surface so as to relieve expansion stress as well as to ensure current collecting efficiency (see, e.g., Japanese Laid-Open Patent Publication No. 2002-83594). In order to increase the adhesion strength between a copper current collector and an amorphous silicon thin-film, Japanese Laid-Open Patent Publication No. 2002-83594 proposes a method for forming a silicon-copper composite layer by forming an amorphous silicon thin-film on the current collector, followed by heat treatment.
In order to prevent an active material from cracking, for example, Japanese Patent Publication No. 2997741 teaches the use of a negative electrode active material composed of SiOx (0<x<2) having a lower expansion coefficient during charge than silicon.
In order to improve battery capacity and cycle characteristics, for example, Japanese Laid-Open Patent Publication No. 2004-047404 teaches the use of a negative electrode active material composed of a conductive silicon composite made of silicon oxide particles, in which silicon microcrystallites are dispersed, covered with a carbon having high conductivity such as graphite.
In order to enhance charge/discharge efficiency, for example, Japanese Patent Publication No. 3520921 teaches a negative electrode having a multilayered structure composed of a carbon layer and a silicon oxide thin-film layer.
However, the above-mentioned prior art references suffer from various problems. For example, the present inventors examined the negative electrode disclosed by Japanese Laid-Open Patent Publication No. 2002-83594 only to find that lithium ion conductivity in the silicon was low, and that polarization increased when high rate charge/discharge was performed and thus the discharge capacity decreased. In a silicon thin film, in particular, a large concentration gradient of lithium is produced in the thickness direction, and the capacity easily decreases. Further, because silicon has an extremely large expansion coefficient, an electrode composed of silicon is highly deformed so that the electrode group is buckled, degrading the battery's characteristics and safety. The term “buckle” used herein is understood to include the following phenomenon: (i) in the case of a spirally-wound electrode group having a circular cross section, the electrode group inwardly curves toward the center thereof due to the expansion of the electrode(s); and (ii) in the case of a spirally-wound electrode group having a rectangular cross section, the electrode group is partially corrugated.
Moreover, the production of the negative electrode disclosed by Japanese Laid-Open Patent Publication No. 2002-83594 involves considerable costs because, in order to relieve the expansion stress at the interface between silicon and current collector, it requires the steps of forming the silicon into a columnar structure as well as performing heat treatment for diffusing copper in the silicon.
As for the negative electrode disclosed by Japanese Patent Publication No. 2997741, because the active material layer is composed of a single-phase SiOx, the conductivity thereof is low. The addition of a conductive material such as carbon to the active material layer is thus required, and therefore the capacity density decreases. Also, because the irreversible capacity is large, some of the lithium ions transferred from the positive electrode to the negative electrode during the initial charge are captured in the negative electrode so that they cannot participate in the charge/discharge reaction. Accordingly, the battery capacity decreases significantly.
Summing up, the negative electrode of Japanese Patent Publication No. 2997741 fails to take advantage of the characteristics of high-capacity silicon and to provide a capacity as expected.
A further problem arises when the negative electrode contains graphite as a conductive material: an electrolyte containing propylene carbonate cannot be used, because a film made of decomposed product of propylene carbonate is formed on the surface of the active material.
As for the negative electrode disclosed by Japanese Laid-Open Patent Publication No. 2004-047404, because SiOx is heat-treated to prepare silicon microcrystallites, it is difficult to control the size of the microcrystallites. In this case, since silicon crystals are inherently produced, it is impossible to form amorphous silicon which is advantageous for absorption and desorption of Li. Besides, such microcrystallites might crack during the expansion if the microcrystallites grow larger than a certain size. Also, because the silicon oxide is covered with graphite, an electrolyte containing propylene carbonate cannot be used as is the case in Japanese Patent Publication No. 2997741.
Moreover, the negative electrodes disclosed by Japanese Patent Publication No. 2997741 and Japanese Laid-Open Patent Publication No. 2004-047404 are produced by mixing the negative electrode active material, a conventional conductive material and a conventional binder to form a mixture which is then applied to a metal foil current collector. In this case, because the active material particles and the current collector are bonded by the binder, the following problem arises: due to the large volume change of the active material during charge/discharge cycles as stated earlier, the conductive material and the binder cannot adjust to the volume change so that during repeated charge/discharge cycles, the contact between the active material and the conductive material as well as that between the active material and the binder cannot be maintained. As a result, the contact between the active material and the current collector is weakened, and the polarization increases, decreasing the charge/discharge capacity.
The negative electrode disclosed by Japanese Patent Publication No. 3520921 contains a silicon oxide in which the oxygen ratio x is set to 0<x≦2. In a thin film layer made of the silicon oxide, the oxygen ratio x is the same in any portion of the layer. When the silicon oxide has a high oxygen ratio, although the expansion coefficient is small during charge and the excellent lithium ion conductivity is obtained, the charge/discharge capacity is small. Conversely, when the silicon oxide has a low oxygen ratio, although the charge/discharge capacity is large, the expansion coefficient during charge is large, and the lithium ion conductivity is low. Moreover, because the silicon oxide thin-film layer is in contact with a carbon layer, the carbon layer and the silicon oxide thin-film layer are separated from each other due to expansion stress during charge, resulting in low current collecting efficiency.
Further, since the production of the carbon layer and the silicon oxide layer requires a completely different production process, the costs for producing negative electrodes will be very high, and negative electrodes cannot be produced efficiently.